This invention relates to longitudinal and angular measuring systems which include a scanning unit movable along a measuring scale for measuring the relative position of two objects. More particularly, this invention relates to an improved error correction apparatus for such systems which is particularly suited for the correction of guiding errors of the two objects. In practice, the two objects may be the bed and carriage of a machine tool or a measuring device, for example.
Errors in processing or measuring machines, particularly errors in the guides, may cause measurement errors. That is, the indicated position as measured by the measuring system may differ from the actual position of the processing or measuring machine by an unacceptable amount. In some cases the indicated position may be consistently greater or less than the actual position. In other cases, the indicated position may range from a value greater than the actual position in some portions of the measurement range to less than the actual position in other portions. The production of highly accurate guide means is difficult and expensive, particularly in the case of large machines. Moreover, wear in the guide means may in time increase the guiding error beyond the admissible tolerances. Various corrective systems have been proposed in the past to compensate or reduce such errors.
For example, in column 1 of the German Auslegeschrift No. 1,477,390, an apparatus is described for compensating guiding errors by regularly sensing these errors in order to exert counter-forces upon the guide means of the machine via an adjustment system. Such compensation systems are generally costly and expensive.
German Offenlegungsschrift No. 2,042,505 discloses an apparatus which uses a correction ruler for error compensation. The correction ruler is scanned via a lever system which readjusts the position of the scanning unit to carry out the desired correction. In many cases correction rulers are difficult and expensive to produce. Moreover, the correction ruler must generally be replaced in time as the error changes with wear. Adjustable correction rulers as shown in German Auslegeschrift No. 1,231,595 are generally expensive as to their design.
In another known compensation method cams are set at certain points along the measuring path where a correction is to be accomplished, and as these cams are traveled over they release a correction signal (German Auslegeschrift No. 1,552,273). The application of corrective cams is also expensive, particularly with large machines.
Yet another error correction method is shown in U.S. Pat. No. 2,988,681, where errors in the mechanics of machines are corrected with the aid of a "correction computer". Several influential factors which relate to measurement errors are stored in the computer, so that an associated correction signal can be used to correct the measured position by an amount which varies with the prevailing position of the measuring system.
A relatively simple correction apparatus is disclosed in the German Offenlegungsschrift No. 2,518,745. There the longitudinal measuring scale is corrected by longitudinal compression or longitudinal stretching to compensate for errors such as machine guiding errors, and thus the indicated position is corrected. One drawback of this method is that it permits compensation of only linear errors.